Networks that utilize optical fiber as a data transmission medium in practice suffer from light attenuation as the light travels down the optical fiber, for example, standard SMF-28™ single mode fiber has an attenuation around 0.2 dB/km at a wavelength of 1550 nm. Therefore, it is common in many networks to incorporate electronic amplifiers at intermediary points along the fiber span. More recently, networks have used optical amplifiers that amplify the light optically without converting the input stream to an electrical signal. There are a number of wavelength ranges of operation in optical networks, principally determined by the low loss transmission spectral regions in glass optical fiber. These ranges have been standardized into a number of bands where two of the most commonly used are the O band (1260-1360 nm, nominally known to be centered around 1310 nm) and the C band (1530-1565 nm, nominally known to be centered around 1550 nm).
The optical fiber commonly used in networks is often of a type that does not preserve the polarization of light that was originally input into the fiber and as such the light that enters an amplifier after propagating along a length of optical fiber can have a random polarization. It is therefore desirable to have optical amplifiers with high optical gain but also low polarization dependence so that the amplifier amplifies the input light by the same amount regardless of the input polarization.
Semiconductor devices are increasingly being used in photonic networks to provide all-optical functionality because of their small size, low cost, capability for bulk manufacture, and ability to be monolithically or hybrid integrated into complex chip designs. A common semiconductor device used to amplify light is a Semiconductor Optical Amplifier (SOA). The SOA usually has a similar structure to a conventional semiconductor laser diode whereby the SOA comprises an optical waveguide to accept input light and output amplified light, a gain medium to provide the optical gain (often termed an active region), and electrodes to pump electric current through the gain medium. The SOA is typically used to receive an input signal from one facet of the device, amplify the input signal and output the amplified signal from either the same device facet (for example a reflective SOA) or a different device facet. An SOA can also be used as an ASE (Amplified Spontaneous Emission) source of light. Typically the gain medium is contained within the optical waveguide. There are many designs of SOA gain medium/waveguides, however those commonly used for high current injection efficiency typically have a buried hetero-structure (BH) waveguide similar to that shown in FIG. 1 or a ridge waveguide (RW) structure similar to that shown in FIG. 2.
A key parameter in the design of an SOA is the minimization of the Polarization Dependent Gain (PDG). To achieve this it is necessary to balance the modal gains (G) for both the transverse electric mode (TE) and transverse magnetic mode (TM). The modal gain for either polarization mode is typically defined as G=g*r, where g is the material gain, and r the confinement factor of the mode for the polarization state. The confinement factor is the proportion of the optical mode that travels in the gain medium. The PDG is the difference between G(TE) and G(TM). Other key parameters in the design of an SOA include low noise and a wide spectral bandwidth of operation. It is also important that the growth conditions and other processing parameters used to make a device with the aforementioned effects such as low PDG are easily controllable and repeatable so that a good device yield can be easily achieved.
P. Doussiere et al., IEEE Photonics Technol. Lett., vol. 6, pp. 170-172, 1994, describes an optical amplifier where the gain medium had a square cross section. The TE and TM confinement factors for an active layer with a square cross section are nominally identical, thus making the gain for both polarization states equal. However, a device with a square gain medium has poor current injection efficiency and thus is generally not suitable for use as an amplifier.
To achieve efficient current injection in both BH and RW designs, it is preferable for the device to comprise a gain medium with an asymmetric cross section wherein the cross sectional width is greater than the thickness. It is also desirable that this cross sectional asymmetry exists over most or all of the SOA waveguide length.
For a gain medium with a wide and thin asymmetric cross section, the confinement factor of the TE mode is substantially larger than that of the TM mode, thus the gain for the TE mode is nominally higher than the TM mode. The exact values of the confinement factors are determined (usually using numerical calculation software) by the waveguide geometry and material refractive indices of the layers used. It is therefore not possible to realize efficient semiconductor devices with low PDG using waveguide geometry alone.
There have been several previous attempts to provide different material gains for the TE and TM modes to compensate for the PDG arising from the cross sectional asymmetry of the gain medium. One approach to equalize the TE and TM gains in an SOA has been described in J. Y. Emery et al., ECOC, vol. 3, PP 165-168 1996 and Patent document U.S. Pat. No. 6,487,007B1. Both documents used an active region comprising a single material layer. Such an active region comprising a single thick layer of material is termed a ‘bulk’ active. These documents used tensile strain in the bulk active region to achieve low PDG. Devices using ‘bulk’ actives generally have low efficiency, high optical losses and require a high injection current.
In contrast to bulk active layers, devices may comprise active regions with one or more layers (with typical thicknesses in the order of <15 nm) that act as quantum wells (QW) that confine electrons and holes to that layer when surrounded by layers of other materials. QW based devices have been found to offer enhanced performance compared to bulk active layers. The improvements that arise by using quantum wells are generally recognized as higher material gain, higher efficiency, lower loss, and lower injection current.
Typically, quantum well-based devices comprise a first layer of material bounded either side by different materials wherein the difference in conduction and valence band edge levels between the first and bounding different materials create a quantum well for any of the electrons or holes in the gain medium. Devices typically comprise one or more (multi) quantum wells (MQW) formed by alternating layers of different materials. The layers in between the layers acting as quantum wells are typically called barrier layers. Typically an MQW structure comprises alternating layers of a first material bordering a second material such that a barrier material layer is disposed between and borders two quantum wells. Barrier material may also be present at the ends of a MQW stack.
The larger the energy difference between the band edge energy levels of the well and the surrounding barrier layers, the stronger the confinement of the holes or electrons. One measure of this confinement strength is termed the ‘offset split ratio’ which is the ratio of energy difference between well/barrier conduction band edges for the electrons to the energy difference between well/barrier valence band edges for the holes.
As shown in FIGS. 3 and 4, the energy levels that light or heavy holes can take in a quantum well are discrete quantized values below the band edge energy level for the particular hole whereas the energy levels of the electrons in a well take discrete quantized values above the band edge energy level for the electrons. To the first order of approximation, g(TE) is primarily generated from the electron-heavy hole transition, while g(TM) is mainly from the electron-light hole transition. For purposes of this application, the heavy hole, light hole and electron wavefunctions shall be termed as the HH-wavefunction, LH-wavefunction and E-wavefunction respectively. The contribution from the E/HH or E/LH transitions to the material gain is determined by the overlap of the E/HH wavefunctions and E/LH wavefunctions respectively. The greater the wavefunction overlap, the greater the contribution to gain for that transition. The Terms ‘heavy’ and ‘light’ holes are known to those skilled in the art and are described in publications such as “Physics of Optoelectronic Devices”, by Shun Lien Chuang, 1995 Wiley, New York). A heavy hole has a larger effective mass than a light hole and is expressed in terms of different set of angular momentum states as a result of different respective positions of the light and heavy holes in a Brillouin zone. Strictly speaking however, the heavy hole and light hole states are mixed states, especially away from the zone center in wave vector, k-space, as such the gain coefficients arising from transitions with from a light hole and a heavy hole are typically numerically calculated.
Previously, the band structure of prior art QW and bulk devices have focused on direct band gap transitions (otherwise known as type-I). Direct band gap transitions occur when no change in momentum is required for a transition between the minimum quantized electron energy level in the conduction band and the maximum quantized hole energy level in the valence band. The term ‘minimum quantized electron energy level’ refers to the quantized electron energy level nearest to the conduction band edge (or first electron level) and ‘maximum quantized hole energy level’ refers to the quantized hole energy level nearest the valence band edge (or first hole level). In an MQW gain medium this has the effect that the minimum quantized electron energy level in the conduction band and the maximum quantized hole energy level in the valence band are spatially located in the same material. FIG. 3 shows an example of the band structure (hole band 23 and electron band 21) of a device where the direct band gap transition occurs in a well 25 between a first heavy/light hole level 24 and a first electron level 22. The well is bordered by a barrier layer 26.
An alternative MQW bandstructure is the indirect (type-II) bandstructure which requires a change in momentum for a transition between the minimum quantized electron energy level in the conduction band and the maximum quantized hole energy level in the valence band. In a MQW gain medium, as shown in FIG. 4 the type II bandstructure (electron band 27 and hole band 29) has the minimum (first electron level) quantized electron energy level 28 in the conduction band in one material (e.g. well layer 31) whilst the maximum quantized hole energy level (first hole level) 30 in the valence band is in the another material (e.g. barrier layer 32).
In comparison with Type-I (direct bandgap) MQW structures, type-II MQWs are commonly thought to provide an unfavorable gain medium for lasers or SOAs, because of low wavefunction overlap between electrons and holes. FIG. 5 diagrammatically shows the first electron wavefunction 38 and first hole wavefunction 36 for a type-II MQW band structure (electron band 33 and hole band 35) with a thick barrier 40. The low wavefunction overlap is due to the electron wavefunction 38 (hence first electron level 34) being confined to individual wells 39, whilst the LH and HH wavefunctions 36 (hence first hole energy level 37) are confined in the barriers 40. J. B. Khurgin et al., IEEE Photonics Technol. Lett., vol. 14, pp. 278-280, 2002 theoretically proposed using a Sb-based strain free Type-II MQW SOA to reduce crosstalk, as a result of longer carrier life time. However, no attempt was made to demonstrate that such a structure can achieve a useful gain level, and also no indication was given on how the design might be used to obtain the low PDG required for most SOA applications.
R. Q. Yang, et al., IEEE J. Quantum Electronics, vol. 38, pp. 559-568, 2002 showed that at longer mid infrared wavelength bands from 3 μm to 5 μm it is possible to use interband tunnelling in InAs—InGaSb based type-II MQWs to realize a semiconductor laser. The MQW design however, for a semiconductor laser does not need to provide a low PDG, and the effect of such a design on low PDG was not disclosed.
The unsuitability of indirect bandgap band structures has led to prior art devices typically using a type-I based band structure, whereby both the heavy and light hole transitions are direct bandgap. MQW gain structures can be grown such that the structure is strained with respect to the substrate the device is grown upon. In principle, any semiconductor layer can be grown upon another semiconductor layer, however in order for the crystal structure to continue through the layers, the lattice constants must sufficiently match. This is difficult to do in practice with semiconductor layers with entirely different constituent elements, therefore it is typical in practice for devices to be grown as a material ‘system’ whereby the majority of the constituent elements in the QW, barrier and other material layers are the same but are formed with different mole fractions.
When different semiconductor layers are grown upon each other, the different material compositions can give rise to a mismatch in lattice constant between the deposited layer and the underlying thick substrate that the gain medium is grown upon. When the lattice constants between the deposited layer and the substrate layer are not matched, the deposited layer becomes either tensile or compressive strained. For an unstrained layer the valence band edges for the light and heavy holes are degenerate, however when strain is applied to the layer the valence band edges for the light and heavy holes split and move away from each other. As the valence band edges for the light and heavy holes change, the corresponding confinement of the respective holes changes. It is thus possible under certain circumstances for the E-HH transition to be type II and E-LH transition to be type I.
Different amounts of strain are introduced by varying the composition of the deposition layer. Introducing strain has the effect of breaking the degeneracy of the LH and HH valence band edge energy levels. Changing the amount and type of strain (tensile or compressive) by changing the layer composition changes the quantized energy levels of the light and heavy holes and the confinement of the LH and HH resulting in a change in the wavefunction overlap. Thus, introducing strain in the QW structure allows the relative gain levels for the TE and TM polarization states to be altered. However introducing increasing amounts of strain increasingly introduces unwanted defects in the layer, changes the bandgap and also the central wavelength peak of operation for the light-hole and heavy-hole transitions.
A number of methods using strained MQWs to realize low PDG SOAs have been reported. One approach using MQW as the gain medium in which barriers are tensile strained and wells are strain free was reported by K. Magari et al. IEEE Photonics Technol. Lett, vol. 2, pp. 556-558, 1990. K. Magari et al. used a gain medium comprising 10.5 nm thick unstrained quantum well layers and 11.5 nm thick barrier layers with −1.7% (tensile) strain. The SOA was 660 μm long and provided 1.0 dB of PDG and 13.0 dB of fiber to fiber gain with an injection current of 200 mA.
An alternative approach reported by M. A. Newkirk et al. IEEE Photonics Technol. Lett., Vol. 4, pp. 406-408, 1993. Newkirk used a gain medium comprising three 3.5 nm thick compressive wells with 1.0% (compressive) strain, three 16 nm thick tensile strained wells with −1.0% (tensile) strain, and seven 10 nm thick, strain free barriers. The SOA was 625 μm long and provided less than 1.0 dB PDG and 4.4 dB fiber to fiber gain with an injection current of 150 mA.
Another approach reported by D. Sigogne et al. ECOC, pp. 267-270, 1995, Electron. Lett., vol. 32, pp. 1403-1405, 1996 used sixteen 8 nm thick 1.1% (compressive) strained wells and sixteen 7 nm thick −0.9% (tensile) strained barriers as the gain medium. The SOA was 940 um long and provided less than 1.0 dB PDG and 23.0 dB fiber to fiber gain with a 150 mA injection current.
The materials used for the wells and barriers in the above prior art documents using Type-I bandstructures were generally based on compositions of InGaAs or InGaAsP grown on an InP substrate, and for operation at a wavelength of 1.55 um.
Devices operating at the 1310 nm wavelength range comprising gain mediums using compositions of In1-x-yAlxGayAs grown on an InP substrate have been described in the prior art. One prior art document describing a laser operating in the 1310 nm wavelength range is described by M. Yamada et al., IEEE, Photonics Technol. Lett., vol. 11, p 164-167, 1999. The bandgap of a gain medium comprising In1-x-yAlxGayAs is very dependent upon the Al mole fraction. It is practically difficult to epitaxially grow gain mediums with bandgaps requiring low Al mole fractions. Therefore, prior art using In1-x-yAlxGayAs quantum wells have concentrated on optical wavelengths close to the 1310 nm band because in this wavelength range, the Al content of the material can be kept above −15% for unstrained layers, thus making the growth easily controllable. Another example of a laser operating at a wavelength range centered around 1310 nm is described by C. Zah et al., IEEE J. Quantum Electronics, vol. 30. p 511-522, 1994. Lasers, however, are designed to be singularly polarized either in the TE or TM mode and as such are designed to have high gain in one polarization and low gain in the orthogonal polarization.
A 1310 nm wavelength low PDG SOA was described by P. Koonath, et al., IEEE Photonics Technol. Lett., vol. 13, pp. 779-781, 2001. In Koonath, the low PDG was realized by introducing 0.33% tensile strain into 3 quantum well layers whilst the barrier layers were strain free.
To make devices operating at longer wavelengths such as the commonly used 1550 nm range (C-band), the amount of Al in the In1-x-yAlxGayAs MQW needs to be greatly reduced such that values for mole fraction ‘x’ are typically around 5%. Under these conditions it is very difficult to grow the InGaAlAs with sufficient control in the Al mole fraction to enable the level of control needed to repeatably obtain a low PDG device using the standard type-I band structure, since the material bandgap becomes very sensitive to the Al mole fraction.
Despite the improved temperature performance of the In1-x-yAlxGayAs lasers and SOAs of the prior art operating in the 1310 nm range, repeatedly manufacturing a low PDG SOA is still problematic because introducing strain to change the heavy and light hole confinement also changes the bandgap of the quantum well. Furthermore, at longer wavelength ranges such as around 1550 nm, the Al mole fraction becomes difficult to manufacturably control.